The present invention relates to electrochemical energy converters with polymer electrolyte membranes, such as fuel cells or electrolyzer cells or stacks of such cells, wherein the cells or stacks comprise adhesively bonded and/or sealed layers.
Electrochemical cells comprising solid polymer electrolyte membranes may be operated as fuel cells wherein a fuel and an oxidant are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes. FIGS. 1-4 collectively illustrate typical designs of a conventional MEA 5, electrochemical cell 10 comprising a PEM layer 2, and a stack 100 of such cells.
Each cell 10 comprises a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) 5 such as that illustrated in an exploded view in FIG. 1. MEA 5 comprises an ion exchange membrane layer 2 interposed between first and second electrode layers 1 and 3, respectively, which are typically porous and electrically conductive, and each of which comprises an electrocatalyst at its interface with the ion exchange membrane layer 2 for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The MEA 5 is typically consolidated as a bonded laminated assembly.
In an individual cell 10, illustrated in an exploded view in FIG. 2, an MEA 5 is interposed between first and second cell separator plates 11 and 12, respectively, which are typically fluid impermeable and electrically conductive. The cell separator plates 11, 12 are typically manufactured from non-metals, such as graphite; from metals, such as certain grades of steel or surface treated metals; or from electrically conductive plastic composite materials.
Fluid flow spaces, such as passages or chambers, are provided between the cell separator plates 11, 12 and the adjacent electrode layers 1, 3 to facilitate access of reactants to the electrode layers and removal of products. Such spaces may, for example, be provided by and the porous structure of the corresponding electrode layers 1, 3. More commonly channels are formed in the adjoining faces of the cell separator plates 11, 12, the electrode layers 1, 3, or both. Cell separator plates 11, 12 comprising such channels are commonly referred to as fluid flow field plates. Resilient gaskets or seals are typically provided around the perimeter of the flow fields between the faces of the MEA 5 and each of the cell separator plates 11, 12 to prevent leakage of fluid reactant and product streams.
Electrochemical cells 10 with ion exchange membrane layers 2 are advantageously stacked to form a stack 100 (see FIG. 4) comprising a plurality of cells disposed between first and second end plates 17, 18. A compression mechanism is typically employed to hold the cells 10 tightly together, to maintain good electrical contact between components, and to compress the seals. In the embodiment illustrated in FIG. 3, each cell 10 comprises a pair of cell separator plates 11, 12, and an MEA 5 interposed therebetween. An alternative configuration has a single separator plate or xe2x80x9cbipolar platexe2x80x9d interposed between pairs of MEAs 5, contacting the cathode of one cell and the anode of the adjacent cell (except for the end cells). The stack 100 may comprise cooling layers interposed between every few cells 10 of the stack, or between each adjacent pair of cells. The cooling layers may be formed within the cell separator plates, for example, or they may comprise channels in bipolar plates used in the stack. Cooling layers of the latter type are disclosed in commonly assigned U.S. Pat. No. 5,230,966.
The illustrated cell elements have openings 30 formed therein which, in the stacked assembly, align to form fluid manifolds for supply and exhaust of reactants and products and, if cooling spaces are provided, for a cooling medium. Again, resilient gaskets or seals are typically provided between the faces of the MEA 5 and each of the cell separator plates 11, 12 around the perimeter of these fluid manifold openings 30 to prevent leakage and intermixing of fluid streams in the operating stack 100.
The present invention relates to apparatus, systems and methods for use in bonding one element of an electrochemical cell stack to another element in the stack and/or for sealing portions of the stack, such as the perimeter of a manifold opening in a cell separator plate. In one embodiment, the inventive method comprises providing a sealing surface on a first element of the stack with a complex groove having a raised portion and a depressed portion. Both the raised portion and the depressed portion lie below the plane of the sealing surface, but the depressed portion is further from the plane than the raised portion. The method then comprises depositing a bead of adhesive on the raised portion, such as by screen printing. When deposited, the bead projects above the plane of the sealing surface. The method then comprises abutting a second element of the stack against the first element. When the first and second elements of the stack are abutted, the adhesive is displaced in part from the raised portion of complex groove, and a portion of the bead of adhesive is received within the depressed portion of the groove.
In another embodiment, the method comprises providing a complex groove having a single raised portion positioned between two depressed portions. The raised portion is again configured to receive the bead of adhesive. When the first element of the stack is abutted with the second element, however, a portion of the bead of adhesive is displaced into each of the two depressed portions.
The present invention is also directed toward an electrochemical cell comprising a membrane electrode assembly positioned between first and second bodies, such as cell separator plates. The second body has a sealing groove with a complex cross-sectional shape. The sealing groove has a shallow portion that is wide enough to receive the volume of adhesive, and a deep portion configured to receive a portion of the adhesive that is displaced during assembly.
In another embodiment, the shallow portion of the sealing groove is curved to increase the strength of the bond between the first body and the second body. The curved shallow portion of the sealing groove is still wide enough to receive the bead of adhesive. Upon assembly, however, the bead is displaced along the entire width of the curved shallow portion. As a result, the bond between the first and second bodies is strengthened to better resist tension and shear.